Integrated circuit fabrication is only one example of a manufacturing process that requires close tolerances of the width of features produced on the wafer. For example, it is important that the width of etched features during processing be controlled within certain critical dimension limits.
Today's wafer fabrication processes include real-time process control steps as a part of the manufacturing process. This permits appropriate adjustments to be made. Ideally, linewidth monitoring is performed in-situ on a wafer-by-wafer basis.
A number of existing techniques may be used to perform linewidth measurements. Techniques based on ordinary microscopes are satisfactory when the line widths are about 1.5 micrometers or larger. For submicron measurements, older methods use scanning electron microscopy. A problem with this method is that it is neither real time nor in-line, and thus is not conducive to wafer by wafer process control.
A recently developed linewidth measurement method uses test patterns in the form of diffraction gratings, which are placed in a test area of the wafer. Depending on whether the substrate being tested is transparent, the incident light is reflected or transmitted to generate diffraction beams. The gratings are monitored during fabrication by illuminating the grating with monochromatic light, such as from laser beam, and analyzing the resulting diffraction pattern to determine its linewidth. If the lines of the grating are made in the same manner as features on the rest of the wafer, measurement of the grating linewidth can be used to infer the sizes of the other features on the wafer.
A linewidth measurement system using a diffraction grating is described in U.S. Pat. No. 4,330,213 to Kleinknecht, et al. The system obtains the intensity of first and second order light and uses the following equation to determine the linewidth, lw: EQU lw=d/.pi. cos.sup.-1 (I2/I1).sup.1/2,
where d is the grating period (linewidth plus spacewidth), and I1 and I2 are measured intensities of the first and second diffraction orders.
The advantage of using intensity ratios is that many factors that affect intensity drop out of the equation. However, a limitation of many existing diffraction methods is that they require the measurement of more than one order of diffraction. The more complex the diffraction model, in the sense of having several unknown variables, the more diffraction orders are needed to solve for linewidth.
The number of observable diffraction orders is a function of the pitch of the grating. Increasing the pitch relative to the wavelength of the light permits a larger number of diffraction orders to be observed. If conventional light sources such as visible or near ultraviolet are to be used, the existing methods require large pitch values to obtain multiple orders. When the required pitch is large relative to the linewidth to be measured, the spacewidth must be wide. However, the use of large spacewidths to create large pitches is detrimental to meaningful measurements because actual devices do not have such configurations.
A need exists for a method of providing multiple diffraction orders for diffraction intensity measurements. The method should provide for in-situ monitoring as a part of wafer process control.